This invention relates to a bag provided with a foldable blow-in port and adapted to be packed with such powdery material as cement.
Cement bags are generally made of several layers of kraft paper. Bags for domestic use comprise three layers whereas bags for export comprise six layers for increasing the strength.
Such cement bag is usually provided with a foldable blow-in port at one corner of its top end as will be described later and the top end and the bottom end of the bag are sewed to close the bag.
In recent years, cement bags are required to have improved strength and moisture proofness because of increased transportation distance and because it is necessary to prevent deterioration of cement while it is stored for a long time.
For this reason, multilayer construction has been used as inner layer kraft paper and as outer layer so-called "cloth-lami" made of a woven cloth of synthetic resin yarn and bonded with a sheet of the same synthetic resin, or so-called "cloth-kraft" comprising the "cloth lami" bonded with a sheet of kraft paper, or a lamination of two synthetic elongated films which have been laminated in orthogonal directions such as Trade name Valeron. When such outer layer is used, it is possible to improve the strength and moisture proofness.
However, the rigidity of the cement bag utilizing such materials is decreased because such materials are relatively soft and pliable so that it is difficult to charge powdery material into the bag. Moreover, as the bag has a small permeability to air when the cement is blown into the bag with air, the air cannot escape through the bag thus making difficult the blow in operation and decreasing the efficiency of handling the packed bag because it expands excessively.
Although it has been tried to increase the rigidity of bag by increasing the number of the inner layers of kraft paper, this solution increases the weight and cost of the bag.
As a result of a number of tests regarding packing of cement in bags, we have found the following facts.
1. The cement blown into the bag together with air accumulates in the bag whereas the air escapes through sewn part of the top and bottom ends of the bag whereby the bag is gradually filled with cement. PA1 2. When the bag is swollen due to a small rigidity the air can not escape thus making it difficult to pack the cement. PA1 3. When the material of the bag is soft the blow-in port which is normally folded in the bag will be projected to the outside due to the internal pressure created in the bag at the time of packing. PA1 5. Where the top portion of the bag is hard, filling operation is especially easy. PA1 6. Especially, the sewn portions at the top and the bottom ends should be reinforced and where reinforcing strips are used at these portions, the number of the layers of the bag may be reduced thus making it possible to reduce the cost. PA1 7. When pin holes are formed through a bag utilizing films of synthetic resinous material for the purpose of discharging air, the moisture proofness is greatly reduced. Moreover, the cement blows out through the pin holes at the time of filling with the result that the operation environment is impaired.
4. When the layer of synthetic material and the layer of kraft paper are not bonded together, the rigidity of the bag is small and the kraft paper which is used as the inner layer would be ruptured by shock since these two materials have different degree of elongation.